Photochromic articles, particularly photochromic plastic materials for optical applications, have been the subject of considerable attention. In particular, photochromic ophthalmic organic glass lenses (e.g., injection molded polycarbonate lenses or CR39 cast lenses) have been commercially desirable because of the weight advantage and impact resistance they offer over glass lenses. Moreover, photochromic transparencies, e.g., photochromic window panes for vehicles such as cars, boats and airplanes, have been of interest because of the potential safety features that such transparencies offer.
The use of polycarbonate ophthalmic lenses, particularly in the United States, is widespread. The demand for sunglasses that are impact resistant has increased as a result of extensive outdoor activity. Materials such as polycarbonate, however, have not historically been considered optimal ophthalmic sunglass lenses with photochromic dyes due to slow activation rates, slow fading (bleaching) rates, and low activation intensities.
Nonetheless, there are several existing methods to incorporate photochromic properties into lenses made from materials such as polycarbonate. One method involves applying to the surface of a lens a coating containing dissolved photochromic compounds. For example, Japanese Patent Application 3-269507 discloses applying a thermoset polyurethane coating containing dissolved photochromic compounds on the surface of a lens. U.S. Pat. No. 6,150,430 similarly discloses a photochromic polyurethane coating for lenses. The content of each of these prior art references is incorporated herein by reference.
Another method involves coating a lens with an imbibing process. A process described in U.K. Pat. No. 2,174,711 or U.S. Pat. No. 4,968,454, both of which are incorporated herein by reference, is used to imbibe a solution containing photochromic compounds into the base coating material. The most commonly used base material is polyurethane.
The two methods described above, however, which involve coating or imbibing the lens after it is molded, have significant shortcomings. For example, typically a coating of about 25 μm or more is needed in order for a sufficient quantity of photochromic compounds to become incorporated into the base of the lens and thereby provide the desired light blocking quality when the compounds are activated. This relatively thick coating is not suited for application on the surface of a segmented, multi-focal lens because an unacceptable segment line and coating thickness nonuniformity around the segment line are produced. The desired surface smoothness is also negatively affected.
Turning to lenses made from injection molded techniques, lenses made of plastic materials such as polycarbonate can be produced by an injection molding process that uses an insert placed in the mold prior to the injection of the molten plastic material (insert-injection molding). The insert can be the means by which photochromic properties are incorporated into the lenses. Insert injection molding is a process whereby the molten plastic resin is injection molded onto an insert having, e.g., a photochromic property, that has been placed in the mold cavity. An example of this process is disclosed in commonly assigned U.S. Pat. No. 6,328,446, which is herein incorporated by reference in its entirety, whereby a photochromic laminate is first placed inside a mold cavity. Molten polycarbonate lens material is next injected into the cavity and fused to the back of the photochromic laminate. This procedure produces a photochromic polycarbonate lens. Because the photochromic function is provided by a thin photochromic layer in the laminate, it is possible to then finish-grind the photochromic polycarbonate lenses with any kind of surface curvature without damaging or degrading the photochromic properties of the lens.
Photochromic lenses can also be made by the cast process as described in U.S. Patent Publication 2007/0122626, the entire contents of which is incorporated by reference. The cast molding process includes placing the photochromic film in a cast mold, then introducing the cast monomer into the mold and then curing the monomer in the mold into lenses either by heat or by radiation.
Resin laminates with photochromic properties that could be considered for use in the above mentioned insert-injection molding technique or the cast molding process have been disclosed in many patents and publications. Examples include Japanese Patent Applications 61-276882, 63-178193, 4-358145, and 9-001716; U.S. Pat. No. 4,889,413; U.S. Patent Publication No. 2002-0197484; and WO 02/093235 (each of which is incorporated by reference herein). The most commonly used structure is a photochromic polyurethane host layer bonded between two transparent resin sheets. Although the use of polyurethane as a photochromic host material is well known, photochromic polyurethane laminates designed especially for making photochromic polycarbonate lenses through, for example, the insert-injection molding method are unique.
Problems associated with conventional insert injection molding techniques in the manufacture of photochromic lens using prior art photochromic polyurethane laminates include polyurethane bleeding and poor replication of lens segment lines. “Bleeding” occurs from the deformation of the polyurethane layer during injection molding processing. In particular, bleeding occurs when the polyurethane layer melts and escapes from its position between the two transparent sheets of the laminate during the high temperature and high pressure injection molding process. The inventors are of the view that bleeding most frequently results, in part, from an excess amount of polyurethane and from using too soft a polyurethane material. The inventors are also of the view that poor replication of segment lines occurs when the layer of polyurethane is too thick and movement of the laminate occurs as pressure from the mold is applied.
In attempts to address at least the bleeding problem, it is preferred to have the polyurethane cross-linked thus making a harder and high temperature resistant polyurethane material. However, cross-linked polyurethane, once made, is difficult to laminate between transparent resin sheets and arrive at a suitable photochromic laminate. For example, a cross-linked polyurethane, once made, is not soluble in a solvent and thus cannot be laminated between transparent resin sheets using a casting method. A cross-linked polyurethane also neither melts nor softens at temperature ranges necessary for making a laminate with transparent resin sheets through the extrusion process. One method that has been considered for incorporating cross-linked polyurethane into a laminate is to start with a liquid polyurethane system such as the one described in U.S. Patent Publication No. 2002/0197484, which is herein incorporated by reference. To make the laminate efficiently, a web coat-laminate line such as the one described in Japan Patent Laid Open 2002-196103, which is herein incorporated by reference, might be used. The coating equipment is capable of coating a uniform layer of liquid polyurethane mixture.
However, this layer will only be partially solidified (or cured) at the moment of in-line lamination. Consequently, any surface defects in the resin sheet and/or the lamination rollers are easily transferred to the soft polyurethane layer during lamination. The most often seen defects in the polyurethane layer include thickness un-evenness across the web and thin spots due to uneven pressure at lamination or improper handling. In order to have the polyurethane layer firm enough to withstand the necessary pressure during lamination and avoid these defects, it needs to first be cured for a certain amount of time. Curing, however, slows down the processing or renders the web coating-laminating approach impossible.
The concepts disclosed in U.S. Patent Publication No. 2005/0233153 (the “'153 Publication”), the entire contents of which are herein incorporated by reference, attempt to address at least some of the problems and shortcomings associated with existing polyurethane laminates. However, the need exists to further address the problems described above, even beyond the teachings of the '153 Publication.